This invention generally relates to coating methods, including processes and materials for use in the manufacturing, repair, and build-up of component surfaces. More particularly, this invention relates to a method of forming a coating by applying a brazing composition containing particles dispersed in a binder, in which the particles are of a material and size to be susceptible to heating and melting when subjected to microwave energy.
Components that operate in a gas turbine environment often require coatings resistant to environmental, thermal, and/or mechanical damage to extend their lives during operation. Coatings are also applied to gas turbine components for dimensional buildup to balance a component or repair its surface. Various coating processes have been developed to deposit metallic and ceramic coating materials capable of surviving and remaining adherent in the chemically and thermally hostile environment of a gas turbine. Examples include thermal spraying, physical vapor deposition (PVD), chemical vapor deposition (CVD), and brazing processes. Thermal spraying, PVD, and CVD processes are generally line-of-sight processes, complicating the coating process and increasing operation cost and time, particularly if not all surfaces of a component are to be coated. In addition, during the coating process the component is typically subjected to intense heat, such as from temperatures required to carry out the process and thermal conduction from the deposited coating. As an example, thermal spray processes, which include combustion flame spray, plasma arc spray, wire arc spray, detonation gun, and high velocity oxygen fuel (HVOF), generally involve propelling a powder or wire feedstock onto a roughened substrate surface while heating the feed stock with a plasma arc, DC arc, or combustion gases. The feed stock may reach temperatures in excess of 3000° C., during which the feed stock at least partially melts before impacting the surface, and thereafter cools and mechanically bonds to the roughened surface to form an adherent coating.
Brazing techniques employed to form coatings include the use of brazing pastes, brazing tapes, and sintered preforms containing or formed from metal alloy powders. In each case, the paste, tape, or preform is applied to a surface to be coated and then heated to a temperature sufficient to melt the alloy, but below the melting point of the substrate being brazed. On cooling, the alloy solidifies to form a permanent metallurgical bond with the substrate. Metal alloys used in brazing processes melt at lower temperatures than the substrates being repaired as a result of containing one or more melting point depressants, such as boron and/or silicon. Otherwise, the alloys typically have compositions similar to the base metal of the substrate being brazed, such that the temperature of the substrate may closely approach its melting temperature during the brazing process. Commonly-assigned U.S. Pat. No. 4,381,944 to Smith, Jr. et al., teaches a more advanced brazing technique in which a powder alloy mixture is prepared containing at least one alloy powder with a composition similar to the alloy being repaired, i.e., a nickel or cobalt-base super alloy, while at least a second alloy powder contains at least one melting point depressant, such as silicon and/or boron. At the braze temperature, the lower-melting second alloy powder melts and flows by capillarity, and carries with it particles of the higher-melting first alloy powder. During the repair cycle, there is at least a partial dissolution of the first alloy powder and the substrate into the molten second alloy, until the composition of the melt is altered enough that its melting point is increased and freezing occurs.
During thermal spray, PVD, CVD, and braze coating processes, the substrate may be heated to temperatures that can adversely affect the mechanical properties of the substrate, such as hardness and fatigue life, as a result of grain growth, incipient melting, recrystallization, or unfavorable phase formation. These processes also have the disadvantage of being high-overhead processes that require significant amounts of time, setup, and expense. An additional disadvantage of brazing techniques that use boron and/or silicon as melting point depressants is the potential for a reduction in mechanical and environmental properties of the resulting coating as a result of the minimal ductility of the borides and silicides they form by reaction with refractory elements. Boron and silicon can also diffuse into the substrate to adversely affect its mechanical and environmental properties.